1. Technical Field
This invention relates generally to catalysts and processes for conducting Fischer-Tropsch synthesis.
2. Background
Synthesis gas, or "syngas," is a mixture consisting primarily of hydrogen and carbon oxides. Syngas is produced during coal gasification. Processes are also well known for obtaining syngas from other hydrocarbons, including natural gas. U.S. Pat. No. 4,423,265 to Chu et al. notes that the major processes for producing syngas depend either upon the partial combustion of the hydrocarbon fuel with an oxygen-containing gas or upon the reaction of the fuel with steam, or on a combination of these two reactions. U.S. Pat. No. 5,324,335 to Benham et al., explains the two primary methods (i.e., steam reforming and partial oxidation) for producing syngas from methane. The entire disclosure of each of U.S. Pat. Nos. 4,423,265 and 5,324,335 is incorporated herein by reference. The Encyclopedia of Chemical Technology, Second Edition, Volume 10, pages 3553-433 (1966), Interscience Publishers, New York, N.Y. and Third Edition, Volume 11, pages 410-446 (1980), John Wiley and Sons, New York, N.Y. is said by Chu et al. to contain an excellent summary of gas manufacture, including the manufacture of synthesis gas.
It has long been recognized that syngas can be converted to liquid hydrocarbons by the catalytic hydrogenation of carbon monoxide. The general chemistry of the much studied Fischer-Tropsch synthesis process is as follows: EQU CO+2H.sub.2 .fwdarw.(--CH.sub.2 --)+H.sub.2 O (1) EQU 2CO+H.sub.2 .fwdarw.(--CH.sub.2 --)+CO.sub.2 (2)
The types and amounts of reaction products, i.e., the lengths of carbon chains, obtained via Fischer-Tropsch synthesis vary dependent upon process kinetics and choice of catalyst.
Many attempts at providing effective catalysts for selectively converting syngas to liquid hydrocarbons have previously been disclosed. U.S. Pat. No. 5,248,701 to Soled et al., presents an overview of relevant prior art. The entire disclosure of U.S. Pat. No. 5,248,701 is incorporated herein by reference.
The two most popular types of catalysts heretofore used in Fischer-Tropsch synthesis are iron-based catalysts and cobalt-based catalysts. U.S. Pat. No. 5,324,335 to Benham et al. discusses the fact that iron-based catalysts, due to their high water gas shift activity, favor the overall reaction shown in (2) above, while cobalt-based catalysts tend to favor the overall reaction of scheme (1).
Recent advances have provided a number of catalysts active in Fischer-Tropsch synthesis. Besides iron and cobalt, other Group VIII metals, particularly ruthenium, are known Fischer-Tropsch catalysts.
The current practice is to support such catalysts on porous inorganic refractory oxides. Particularly preferred supports include silica, alumina, silica-alumina, and titania. In addition, other refractory oxides selected from Groups III, IV, V, VI and VIII may be used as catalyst supports.
The prevailing practice is also to add promoters to the supported catalyst. Promoters can include ruthenium (when not used as the primary catalyst component), rhenium, hafnium, cerium, and zirconium. Promoters are known to increase the activity of the catalyst, sometimes rendering the catalyst three to four times as active as its unpromoted counterpart.
Contemporary cobalt catalysts are typically prepared by impregnation of the catalytic material upon the support. As described in U.S. Pat. No. 5,252,613 to Chang et al., a typical catalyst preparation may involve impregnation, by incipient wetness or other known techniques of, for example, a cobalt nitrate salt onto a titania, silica or alumina support, optionally followed or preceded by impregnation with a promoter material. Excess liquid is removed and the catalyst precursor is dried. Following drying or as a continuation thereof, the catalyst is calcined to convert the salt or compound to its corresponding oxide(s). The oxide is then reduced by treatment with hydrogen or a hydrogen-containing gas for a period of time sufficient to substantially reduce the oxide to the elemental or catalytic form of the metal. U.S. Pat. No. 5,498,638 to Long points to U.S. Pat. Nos. 4,673,993, 4,717,702, 4,477,595, 4,663,305, 4,822,824, 5,036,032, 5,140,050, and 5,292,705 as disclosing well known catalyst preparation techniques.
The entire disclosure of each of the U.S. patents mentioned in the previous paragraph is incorporated herein by reference.
Fischer-Tropsch synthesis has primarily been conducted in fixed bed reactors, gas-solid reactors, and gas-entrained fluidized bed reactors, fixed bed reactors being the most utilized. U.S. Pat. No. 4,670,472 to Dyer et al. provides a bibliography of several references describing these systems. The entire disclosure of U.S. Pat. No. 4,670,472 is incorporated herein by reference.
More recently, however, attention has been directed to conducting Fischer-Tropsch synthesis in three-phase slurry reactors. Three phase reactions involve the introduction of a fluidizing gas into a reactor containing catalyst particles slurried in a liquid. Particularly useful in Fischer-Tropsch processes is the slurry bubble column reactor (SBCR). In a SBCR, catalyst particles are slurried in liquid hydrocarbons within a reactor chamber, typically a tall column. Syngas is then introduced at the bottom of the column through a distributor plate, which produces small gas bubbles. The gas bubbles migrate up and through the column, causing a beneficial turbulence, while reacting in the presence of the catalyst to produce liquid and gaseous hydrocarbon products. Gaseous products are captured at the top of the SBCR, while liquid products are recovered through a filter which separates the liquid hydrocarbons from the catalyst fines. U.S. Pat. Nos. 4,684,756, 4,788,222, 5,157,054, 5,348,982, and 5,527,473 reference this type of system and provide citations to pertinent patent and literature art. The entire disclosure of each of these patents is incorporated herein by reference.
Using a SBCR to conduct Fischer-Tropsch synthesis has certain recognized advantages. As noted by Rice et al. in U.S. Pat. No. 4,788,222, advantages of a slurry process over a fixed bed process include better control of the exothermic heat produced during the reactions and better maintenance of catalyst activity by allowing continuous recycling, recovery and rejuvenation procedures to be implemented. U.S. Pat. Nos. 5,157,054, 5,348,982, and 5,527,473 also discuss advantages flowing from the use of a SBCR.
Heretofore, catalyst particle size has not been deemed to be a critical parameter in SBCRs. It is desired that the catalyst particle be reasonably filterable, but also easily dispersible. The art suggests that the particle sizes of 1-200 microns meet these requirements. (See Chang, Col. 5).
Notwithstanding the research and development heretofore conducted, Fischer-Tropsch synthesis in a three phase slurry bubble column reactor is by no means a refined procedure. The process remains expensive, owing in part to the significant cost of promoted catalysts in the current state of the art. Environmental concerns also come into play, not only with respect to the operation of a SBCR, but also with regard to the preparation of catalysts, which involves the use of organic solvents.
The present invention encompasses certain discoveries that have resulted in a more rate efficient, more selective, environmentally friendly, and more cost efficient process for conducting Fischer-Tropsch synthesis, particularly in a slurry bubble column reactor.